Biology Reference
In-Depth Information
Inorderforabirdtosoarrather thantoflap,itsweight mustbelowrelative tothesur-
face area of the wings and tail, giving it a low
wing loading.
Birds with low wing loading
include cranes, hawks, vultures, anhingas, pelicans, and many others. Loons are an ex-
ampleofbirdswithhighwingloading—thatis,theirbodyweightisveryhighrelativeto
the surface area of their wings. If a loon is missing just a couple of flight feathers, flight
becomes difficult or impossible. Penguins have such extremely high wing loading that
theirsmallwingscan'tgetthemoffthegroundatall.Hummingbirdsandmanysongbirds
have high wing loading and must keep flapping to stay aloft.
The shape of the wings determines how a bird soars. An albatross, with its extremely
long, slender wings, has a high
aspect ratio,
which allows it to fly at high speeds at low
altitudes (sometimes barely above the ocean's waves), while being extremely maneuver-
able. A condor's wide, slotted wings give it a low aspect ratio. Birds with a low aspect
ratio are adapted to slow speed, high altitude, effortless flight. Birds such as these, with
aspect ratios at either extreme, may have difficulty taking off from the ground without
facing a stiff headwind. Most soaring birds fall somewhere between these extremes.
Q How can owls fly so silently?
A
Given their body size, owls have very large wings, allowing them to flap more slowly
than many birds, reducing noise. The velvety surface of their flight feathers, along with
their fluffy body plumage, helps absorb sound waves. The velvety surface also muffles
any scraping sounds as the feathers slide against each other in flight.
But the one feature that is probably most significant in creating an owl's silent flight
is the stiff,comblike fringe along the edge ofthe outer vanes ofcertain wing feathers, the
firstprimaryfeathers.Thissoftensthecontactbetweentheairandtheleadingedgeofthe
wing, essentially breaking up the
whoosh
of a wing beat into 50 or 100 tiny
whooshes
.
MADE FOR FLIGHT
Birds are masters of flight, and because we can easily see their wings and feathers, many
people assume that those are the key features that enable birds to fly. But birds are ac-
tually built for flight on the inside, too, including their skeletons. The bones of birds are
exceedingly light: most flying birds have skeletons that weigh just half or a third of the
weight of their feathers. Flying birds have huge airspaces in many or most of their bones,
although some bones in each bird are filled with enough bone marrow to manufacture
blood cells. The long wing bones of vultures and swans are hollow enough that in earlier
